Only Buy Official Sony Memory Cards.
From what i've been told and also from my experiences is that fakes only hold about half the data, even though the may say more on your PSP. Everything above half goes corrupt, I've learned from experience and other reports. I wouldn't get fakes, the sellers have positive feedback because people leave positive when they get the item, and find out a month later that its defective. I have found out from various people that the Memory Cards, unless marked by Sony are fake, and are likely to corrupt and even damage your PSP.
Most eBay sellers and buyers are looking out for cheap sales and as such are subject to buying these fakes from eBay sellers and even buyers believe wow this is good value whereas the item was clearly fake and not as described. Items should only have the word Sony on their memory cards or the appropriate company, and include the MagicGate logo on the top right hand corner:
If you look through the items, you will clearly see that most of the items are not endorsed or even supported by Sony, it says no sign of Official or Real, and if Sony does not appear in the corner like this:
Your memory card is FAKE.
DO NOT BUY ANY MEMORY CARD FOR YOUR PSP UNLESS IT'S FROM SONY OR A GUARANTEED SELLER!
The only exception is SanDisk who are supported by Sony.